The present invention relates to a friction stir welding method and to a joint shape member and a welding method for the producing the same. For example, the present invention relates to a friction stir welding method which is suitable for use in welding members made of an aluminum alloy, such as members employed in forming a car body of a railway car and other structural bodies.
A friction stir welding method is a method in which, by rotating a round-shaped rod (hereinafter referred to as a rotary body), inserting it in a joint between members, and then moving the rotary body along the joint line, the joint is heated, and the material is softened and plastically flows, so that the members are solid-phase joined.
The rotary body comprises a tool having a small diameter portion which is inserted in the joint and a large diameter portion which is positioned outside the joint. The small diameter portion and the large diameter portion are positioned on the same axis. The large diameter portion is rotated. A boundary portion between the small diameter portion and the large diameter portion can be inserted a little into the joint region during the welding.
A welded joint produced by a friction stir welding method can be used to weld abutting members and overlapping members to be joined. The above stated technique is disclosed, for example, in Japanese patent announcement laid-open publication No. 505090 (EP 0615480 B1) and "Welding & Metal Fabrication" (January 1995), pages 13-14 and 16.
Japanese patent laid-open publication No. Hei 9309164 (laid-open date: Dec. 2, 1997) (EP 0797043 A2 (laid-open date: Sep. 24, 1997) relates to a friction stir welding method for welding a hollow member and discloses the welding of a center rib for connecting two sheets using friction stir welding. Further, a concave portion is provided on the sheet face of the joint. Further, this Japanese patent laid-open publication was laid-open after the filing date of a first application (Japanese patent application No. Hei 9-196761 (application date: Jul. 23, 1997) of the present application.
In a friction stir welding method, since the rotary body operating as a joining tool is inserted into the welding zone of the member to be welded, a large force is applied to the member to be welded. In contrast, in a conventional arc welding method, since the welded portion is fused and welded, a force such as occurs during friction welding is not generated. For this reason, in the case where the friction stir welding method is applied to join I-shaped members, which were previously joined by the conventional arc melting method, since a separation occurs in the joint between the members being welded, a good quality weld can not be obtained.
The above circumstances will be explained more specifically with reference to FIG. 18 to FIG. 20. Two members 310 and 311 to be welded (for example, members made of an aluminum alloy) have substantially the same thickness at the edges where they are to be joined and are in parallel with each other, and further have vertical abutting faces 313. Under a condition in which the abutting faces 313 are in contact with each other and in which the two members 310 and 311 are fixed to a bed stand 40, the friction stir welding method is carried out to weld the members 310 and 311 together.
By rotating a rotary body 20, which serves as a welding tool and is comprised of a round shaped rod having a concave shaped small diameter portion 21 at a lower end thereof, the rotating concave shaped small diameter portion 21 can be inserted into the joint between the members. Under this condition, the rotary body 20 is moved along the joint between the abutting faces 313 of the two members 310 and 311, causing the two members 310 and 311 to be welded.
A substantially flat boundary portion 20b, formed between the small diameter portion 21 and a large diameter portion 20a, limits the distance that the small diameter portion 21 can extend into the two members 310 and 311. Since the length of the small diameter portion 21 is smaller than the thickness of the two members 310 and 311, the welding is carried out only on the upper face of the two members 310 and 311. Namely, a non-penetration type friction stir welding method is carried out.
In FIG. 19, in the vicinity of the rotary body 20, a plastic flow condition exists. A plastic flow of material in a plastic flow area 30 attempts to discharge to the surface, but such movement, for the most part, is restrained by the boundary portion 20b. As a result, at the abutting faces 313 of the two members 310 and 311, a force is applied in a downward direction and a lateral direction. Accordingly, the plastic flow of material in the plastic flow area 30 is pushed downward toward a lower portion of the joint between the abutting faces 313, so that the space between the abutting faces 313 is enlarged by the lateral force.
As shown in FIG. 20, the plastic flow of material 31 flows downward from the lower portion of the plastic flow area 30, and an enlarged clearance 313s is produced at the lower portion. Further, the volume of the plastic flow material 31 which flows downward from the plastic flow area 30 is insufficient to fill the clearance, and so voids 380 representing a defect are created in the plastic flow area 30. Therefore, a normal weld can not be produced. Further, the plastic flow area shows a substantial welding bead 30.
The inventors of the present invention have discovered the above stated phenomenon through various experiments. In the above stated non-penetration type welding method on I-shaped joint shape members, in a case where a clearance formed between the abutting faces 313 before the welding is more than 0.2 mm, it was found that a good welding can not carried out.
Further, friction stir welding is carried out on the basis of a premise that the clearance formed between the abutting faces 313 is extremely small, as stated above. A large scale structural body, such as a car body of a railway car, in a case where friction stir welding is carried out on extruded frame members of about 25 m in length, the clearance becomes large due to inaccuracies in the extruded frame member, and, as a result, it is difficult to carry out welding on such members.
Further, in a case where the welding is carried out on hollow members, the face of the sheet becomes dented, and, therefore, it is impossible to carry out a good welding. Further, at a start point of the welding, it is difficult to carry out a good welding. Also, at a finish point of the welding, it is difficult to carry out a good welding.